Manufacture of cellulosic products



United States Patent '2 994 634 MANUFACTURE OF CE LLUL'0SIC PRODUCTSJack E. Jayne, Menasha, Wis., assignor to Kimberly- Clark Corporation,Neenah, Wis., a corporation of Delaware Filed Jan. 2, 1958, Ser. No.706,568

No Drawing.

8 Claims. (Cl. 162-138) The present invention is concerned With themanufacture of cellulosic products. More particularly it relates tocyanoethylcellulose products and process for their manufacture.

Although the paper prepared from natural cellulose is one of the mostversatile and inexpensive industrial products, it has been recognizedthat cellulose chemically modified could be made into paper of greatlyimproved characteristics. One modification which has been examined hasbeen the cyanoethylation of cellulose, or the introduction of thecyanoethyl group, CH CH CN, into the glucose unit of the cellulosemolecule by treatment of the cellulose with acrylonitrile. It has beenfound that the cyanoethyl group can be substituted into the glucose unitof the cellulose molecule at one or more of the hydroxyl positions bythe substitution of the cyanoethyl group for the hydrogen of thehydroxyl group in an ether type of bond. The partial cyanoethylation ofcotton textile fibers has resulted in a new type of cotton textile withgreatly improved resistance to micro-organism attack, to wet and dryheat degradation and to abrasion. The formation of paper fromcyanoethylated cellulose fiber, however, has been found to be much moredifiicult principally because of the problem of bonding thecyanoethyl'ated cellulose fibers into a paper sheet.

A simplified flow diagram of the process for cyanoethyl ating cellulosefibers and bonding the cyanoethylated cellulose fibers obtained therebyinto a paper sheet in accordance with this invention is as follows:

Cellulose Fibers Disperse in Aqueous Solution of Basic Hydroxide of 510%Concentration Add Acrylonitrile in an Amount of about 85-300 percent byweight of cellulose, but less than amount of water present in hydroxidesolution React at Temp. of Less Than 50 0. to degree of Substitution of0.5-1.5 Oyanoethyl Groups per Glucose Unit of Cellulose RedisperseSubstituted Cellulose in Water and Form on Paper Machine Heat BondFormed Sheet While Retaining Moisture Content of at least l l PressMoist Sheet or I Laminated Sheets i at Between 275 and 475 F.

In the manufacture of paper from cellulosic raw materials the cellulosicfibers, after they have been freed from lignin or other material withwhich they are associated in nature, are usually beaten in an aqueoussuspension to fray the fibers and expose a large surface area of fibrilsor micro-fibrils. This is part of the process usually referred to in thepaper industry as hydration of the pulp. A very dilute suspension of thebeaten fiber is then prepared and flowed onto a fine wire screen orforming wire to form a thin layer of cellulose fibers. As water drainsaway from the layer of fibers on the wire the layer is converted into aweak fibrous web or sheet which is stripped from the wire onto a feltblanket when the fiber content of the web is 17 to 20 percent in atypical case. The web is then conveyed through a press section whichpresses water out of the sheet to a water content in a typical case of67 percent. The paper may then be further dried in a dryer section (to awater content of 5 to 15 percent) by pressing and contact with heatedrolls. The strength of the paper web is a function of the dryness andthere is no difference in the strength of paper dried at an elevatedtemperature or room temperature.

The strength of .a moist paper web on the forming wire depends somewhatupon the surface tension of the Water content of the web. There is,however, a wide variation between the strengths of webs of various typesof cellulose fibers. A web of a groundwood pulp having a moisturecontent of percent may have a strength as measured by the breakinglength in meters of 27, while a bleached sulfite pulp web of the samemoisture content may have a breaking length of 106 meters. A web havinga strength of 27 meters, however, has insufficient strength to be formedinto paper by typical papermaking machinery and groundwood pulp is.usually mixed with at least 25 percent of a strong pulp such as kraft orsulfite to give it additional strength. As the water content of thepaper sheet in the press and dryer sections is reduced below about 60percent water the reduction in strength from surface tension effects isreplaced by an interfiber bonding between the cellulose fibers believedto be one of secondary valence or molecular cohesion between hydroxylgroups of adjacent fibrillae. The dry paper sheet will normally have astrength at least several times that of the wet web.

The strength characteristics of a layero f cyanoethylated cellulosefibers having a degree of substitution of 0.5 or greater, developedduring drying are quite different than those of a cellulose web. The wetweb has very little strength and even in a typical cyanoethylated sheetwhich has been dried by drainage and pressed to a water content of 44percent the strength may only have developed to a breaking length inmeters of 4.2. This strength is too low to permit the removal of thisweb from the wire and further handling by conventional papermakingapparatus. Furthermore, upon complete air drying of the sheet to normalair dry moisture content, the sheet will lose substantially allstrength. Since the strength of the formed paper sheets depends upon thehydroxyl group it is apparent that cyanoethylated cellulose fibers inwhich any substantial amounts of the hydroxyl group have beensubstituted by cyanoethyl groups will have little tendency to bond withother such fibers by the typical paper forming bonds to form a papersheet.

In addition to the problems of making paper of cyanoethylated cellulose,it has also been found that the method of cyanoethylation of thecellulose and the extent to which it is carried out may greatly affectthe resulting product.

For example, under certain reaction conditions the terminal cyanoportion of the substituted cyanoethyl group may be hydrolyzed so as tobecome converted into the carboxyethyl group which will have aconsiderable effect upon the properties of the cyanoethylated cellulose.Furthermore, if cellulose fibers are cyanoethylated so that the degreeof substitution (D.S.) of the hydroxyl groups of a glucose untit bycyanoethyl groups is increased to the 2-3 range the resultant product issoluble in organic solvents and tends to lose its fibrous character ifcontacted with organic solvents. Thus the cyanoethylated cellulose maydissolve in the acrylonitrile used to cyanoethylate the cellulose. Thesuitability of cyanoethylated cellulose for particular uses is thereforestrongly dependent upon the manner in which the cellulose iscyanoethylated.

It is an object of the present invention to provide a method ofmanufacturing a fibrous cyanoethyl cellulose pulp having a degree ofsubstitution between about 0.5 and 1.5 and suitable for the manufactureof paperlike sheets.

Additional objects of the invention will be apparent from the followingdescription.

In accordance with the process of the present invention it has beenfound that cyanoethylated cellulose fibers having between about 0.5 and1.5 cyanoethyl groups per glucose unit, in a form particularly suitablefor the preparation of durable paper sheets, can be formed by treatingcellulosic fibers in an aqueous solution of about 5 to percent sodiumhydroxide, then adding acrylonitrile to the mixture in an amount ofabout 85-300 percent of the weight of the cellulose fibers, reacting themixture until a desired degree of substitution is attained whilemaintaining the mixture at a temperature of less than about 50 C., thenseparating the fibers from the solution.

The cellulosic raw material which is converted into cyanoethylatedfibrous cellulose by the process of the present invention can be anytype of fibrous cellulose suitable for the manufacture of paper, such aswood pulp, cotton fibers, esparto fibers, bagasse fibers and othertypical cellulosic fibers. The wood pulp may be prepared by conventionalmethods such as the kraft and sulfite processes and may be bleached orunbleached. The cellulose fibers are first treated in an aqueousalkaline bath of a strongly basic hydroxide having a hydroxideconcentration of between about 5 and 10 percent. Sodium hydroxide is thepreferred alkaline material although other water soluble strongly basichydroxides such as the other alkali metal hydroxides and strongly basicquaternary ammonium hydroxides may be used. The alkaline treatment ofthe cellulose fibers apparently swells the fibers so that a largesurface area is available for the cyanoethylation reaction. The fibersshould be thoroughly contacted with the alkaline solution and tenminutes agitation of the cellulose fibers in the alkaline solution isusually sufficient. The ratio of pulp to the aqueous hydroxide issubject to considerable variation based upon the other conditionsemployed in the hydroxide treatment, such as the time and temperature ofthe treatment. Drastic conditions are avoided in order to minimizedegradation of the cellulosic material. The ratio of cellulose toalkaline solution is not critical and it has been found that a treatmentof one part of pulp per nine parts of alkaline solution for a period ofabout 10 or more minutes at a temperature of the order of roomtemperature, is generally suitable.

After completion of the treatment of the cellulose fibers with thealkaline solution the acrylonitrile can be added directly to the mixtureof cellulose fibers and alkaline solution. It has been found that theaddition of acrylonitrile in an amount of between about 85 and 300percent by weight of the cellulose fibers furnishes a reaction solutionwhich can be most conveniently employed to obtain cyanoethylatedcellulose having the desired degree of substitution of 0.5 to 1.5. Thereaction between cellulose and the acrylonitrile is exothermic. It hasbeen found that if the temperature of the reaction mixture is permittedto rise above about 50 C., excessive hydrolysis may occur resulting in acyanoethylated cellulose containing an excessive proportion of carboxylgroups. It may therefore be necessary to cool the reaction mixture tomaintain it within the desired temperature range. The cyanoethylatedcellulose pulp is removed from the reaction mixture when the pulp has adegree of substitution of cyanoethyl groups for hydroxyl groups perglucose unit of the cellulose between about 0.5 and 1.5. This isequivalent to a nitrogen content of the cyanoethylated cellulose ofabout 3.72-8.75 percent. The pulp is then washed with water to free itof all excess alkaline material and acrylonitrile. Thorough washing isparticularly important if the cyanoethylated pulp is to be made intoelectrical grade paper. It has been found that cyanoethylated pulpprepared in this manner has a low carboxyl content and is particularlysuitable for use in the preparation of dielectric materials.

Now that the process of cyanoethylation of cellulosic fibers of thepresent invention has been generally described the process may befurther illustrated by the following examples. Unless otherwisespecified, all parts are by weight.

EXAMPLE 1 One hundred parts by weight (on an oven dry basis) of ableached kraft made from northern softwood, predominately spruce, wasintroduced into 880 parts of an aqueous 9.4 percent sodium hydroxidesolution. The pulp was slurried in the solution for 10 minutes at roomtemperature. Two hundred and seventy-five parts of acrylonitrile werethen added to the solution and the resultant reaction mixture stirredfor 70 minutes. The temperature of the reaction mixture was controlledso that the temperature did not rise above 48 C. The pulp was thenseparated from the reaction mixture and thoroughly washed with wateruntil all caustic and acrylonitrile were removed. The resultant pulp wasanalyzed for nitrogen and found to contain 7.52 percent nitrogen,equivalent to a D.S. of 1.19.

EXAMPLE 2 Two hundred and five parts of unbleached kraft pulp wasintroduced into 1751 parts of an aqueous 8.6 percent sodium hydroxidesolution. The pulp was slurried in the solution for 10 minutes at roomtemperature. Five hundred and fifty parts of acrylonitrile was thenadded to the slurry to form the reaction mixture. This reaction mixturewas agitated for minutes while the temperature was not permitted to riseabove 49 C. The resultant cyanoethylated pulp was removed from thesolution and washed. Analysis of the pulp disclosed that the pulp had anitrogen content of 8.24 percent, equivalent to a D.S. of 1.37.

EXAMPLE 3 Twenty-four parts of commercial bleached kraft wood pulp wasslurried with 156 parts of 10.0 percent sodium hydroxide for 10 minutesat room temperature. Fifteen parts of acrylonitrile was then added tothe slurry and the resultant reaction mixture agitated for 90 minutes ata temperature of not greater than 27.5 C. The cyanoethylated pulp wasremoved from the reaction mixture and thoroughly washed. The resultantcyanoethylated pulp on analysis was found to contain 3.38 percentnitrogen, equivalent to a D.S. of 0.45. This pulp was a doughygelatinous pulp which tended to be resinous or pasty rather than fibrousin character and was unsuitable for conversion into paper.

EXAMPLE 4 Twenty-four parts of commercial bleached kraft wood pulp wasslurried with 156 parts of 10 percent sodium hydroxide for minutes atroom temperature. Twentyone parts of acrylonitrile was then added to theslurry and the resultant reaction mixture agitated for 90 minutes at atemperature not greater than 27.5 C. The cyanoethylated pulp was removedfrom the reaction mixture and thoroughly washed. The resultantcyanoethylated pulp on analysis was found to contain 5.49 percentnitrogen equivalent to a D8. of 0.80. This product had the typicalfibrous appearance and was suitable for papermaking according to theprocess of the present invention.

EXAMPLE 5 Seventy-three parts of commercial bleached kraft wood pulp wasslurried with 552 parts of 8.0 percent sodium hydroxide for 10 minutesat room temperature. Ninetynine parts of acrylonitrile was then added tothe slurry and the resultant reaction mixture agitated for 90 minutes ata maximum temperature of 28 C. The pulp was then separated from themixture and thoroughly washed. Upon analysis the pulp was found tocontain 6.37 percent nitrogen, equivalent to a BS. of 0.96 and acarboxyl content of 0.14 percent.

EXAMPLE 6 Seventy-three parts of commercial bleached kraft pulp wasslurried in 600 parts of a 6.0 percent sodium hydroxide solution for 10minutes. Ninety-nine parts of acrylonitrile was then added to the slurryand the result ant mixture agitated for 90 minutes at a maximumtemperature of 28 C. The pulp was then removed from a mixture andthoroughly washed. An analysis of the pulp showed that the nitrogencontent was 4.36 percent, equivalent to a D.S. of 0.60 and the carboxylcontent was 0.20 percent.

EXAMPLE 7 Two hundred and five parts of unbleached kraft pulp wasslurried with 1759 parts of an 8.6 percent sodium hydroxide solution atroom temperature for 30 minutes. Two hundred and seventy parts ofacrylonitrile was then added to the mixture and reacted with agitationfor one hour at a maximum temperature of 325 C. The resultant pulp wasremoved from the mixture and thoroughly washed. The pulp was thenanalyzed and found to contain 6.76 percent nitrogen, equivalent to a D5.of 1.05 and to have a carboxyl content of 0.33 percent.

It has been found in accordance with the process of the presentinvention that cyanoethylated cellulose fibers can be formed into asheet and bonded together to produce a cyanoethylated paper which has awet strength equivalent to that of conventional cellulose papers andwhich is sufficient to permit the cyanoethylated cellulose sheet to behandled in the same manner as conventional cellulosic webs. Broadly, theprocess comprises forming a layer of the cyanoethylated cellulose fibersand then heating the layer under moist conditions until bonding takesplace. The strength of a moist web of cyanoethylated cellulose fibersmay be increased in a typical web from a breaking length in meters of 4meters to a breaking length in meters of 50 meters. It is thus apparentthat a web of moist cyanoethylated cellulose fibers too weak to beremoved from the forming wire by conventional papermaking techniques canbe bonded by this process into a web having a strength equivalent toconventional cellulose moist webs and capable of being removed from theforming wire and further treated by the conventional methods ofpapermaking.

While the process is particularly applicable to the formation of a papersheet from cyanoethylated cellulose fibers having a degree ofsubstitution between about 0.5 and 1.5 it may be applied to theformation of a bonded sheet of cyanoethylated cellulose fibers having agreater or lesser degree of substitution as long as the cellulose hasretained its original fibrous character.

The cyanoethylated cellulose fibers can be prepared for forming intosheets and formed into the unbonded web on a paper forming wire in muchthe same manner as cellulose fibers are treated in the forming ofconventional papers. The fiber lengths of the fibers should besubstantially the same as the corresponding lengths of cellulose fibersused in the manufacture of paper. If there are any fiber clumps, theseshould be broken up or dispersed by a defibering action. Thecyanoethylated pulp need not be beaten however in the usual manner ofbeating cellulose pulps to increase the fibrils exposed and causehydration of the pulp if the pulp has a DS greater than 0.5. It has beenfound that beating has substantially no efiect upon the bond formingtendencies of the cyanoethylated cellulose fibers.

The defibered cyanoethylated cellulose pulp is dispersed in an aqueousphase to form a pulp stock having a consistency of approximately 0.051.5percent in substantially the same manner as conventional cellulose pulpis formed into a stock. The term consistency is used to mean thepercentage by weight of oven dry pulp in a combination of pulp andwater. The stock is then flowed onto a mechanical support or paperforming wire to form a uniform layer of cyanoethylated cellulose fibers.This can be accomplished in the same manner as the layer of cellulosefibers is formed in a conventional papermaking operation, for example,on a Fourdrinier machine or a cylinder machine.

The heating of the cyanoethylated cellulose pulp layer in the presenceof moisture, very rapidly develops the necessary strength to form thelayer into a self-sustaining web. Various methods of applying heat to amoist layer of fibers may be utilized. A preferred method employs steamas the heating agent. Low pressure, low temperature steam, will bond amoist layer of the cyanoethylated cellulose fibers into a sheet within avery short time, for example 4 to 12 seconds. The steam may be appliedto the layer of cyanoethylated fiber on the forming wire by conventionalmethods of steaming fiber webs. Other conventional methods of applyingheat such as heated rollers, infra red heaters, etc., may also beemployed to apply the heat to the cyanoethylated cellulose fiber layer.The heat source should be capable of raising the temperature of themoisture in the web to about the boiling point. Lower temperatures forexample F., may be used but corresponding longer time to complete thebonding will then be required.

The layer of fibers may be formed by air forming or other conventionalmethods of forming fiber layers as Well as by water forming but in thiscase the layer should be moistened prior to the application of the heat.The moisture content of the web when heat is applied must be at leastabout 10 percent in order for bonding to take place and is preferablygreater than 15 percent. When the web is water formed on the wire, themoisture content will normally be much greater. Bonding, however, isequally eitective in webs having high moisture contents, although it maytake a longer heating period to raise the fibers to the bondingtemperature because of the amount of moisture present.

Because of the extreme weakness of the unbonded cyanoethylated celluloseweb, the web is normally heated on the forming wire to effect bonding.However, the web may be bonded at any stage of the papermaking operationif the conditions of heat and moisture content are complied with and thenature of the support is not critical.

In a normal papermaking operation the paper web is removed from aforming wire at a consistency of about 17 percent. The cyanoethylatedcellulose web of the present invention retains water with less tenacitythan normal paper web and the drying action of the applied heat furthertends to decrease the water content of the web on the wire so that thecyanoethylated bonded paper sheet will normally leave the forming wireat a lower consistency than a corresponding paper web. Therefore, thedrying operation which normally follows the forming operation withconventional paper can be greatly de- 7 creased with the cyanoethylatedcellulose web or even omitted entirely.

The bonding process of the present invention may be applied to thebonding of cyanoethylated cellulose fibers mixed with other fibers, intowebs of sufiicient strength to be handled and made into paper. It isparticularly applicable to forming webs of cyanoethylated cellulosemixed with the non-hydrating synthetic fibers such as glass,polyacrylonitriles, copolymers of polyvinylidene chloride andpolyvinylchloride, polyethylene terephthalates, nylons, polyurethanesand the non-hydrating rayons. These hydrophobic fibers are not bonded bydrying of the web, so that ordinarily they are made into bonded webs bythe application of adhesives. The bonds developed in thecyanoethylcellulose fibers by the method of the present invention aresufficiently strong that webs of a mixture of a synthetic fiber andcyanoethylated cellulose containing as little as percentcyanoethylcellulose can be bonded into a self-sustaining web.Non-fibrous additives may also be incorporated into thecyanoethylcellulose webs.

For example, a highly decorative paper may be made by mixing as much as50 percent glass flakes with cyanoethylcellulose fibers, forming a layerof the mixture and bonding the layer into a web in accordance with theprocess of the present invention.

The formation of cyanoethylated cellulose sheets may be furtherillustrated by the following examples.

EXAMPLE 8 Fifteen hundred parts of commercial wood pulp was slurriedwith 13,500 parts of 8.6 percent sodium hydroxide solution for 15minutes. Acrylonitrile in an amount of 3,859 parts was then added to theslurry over a 15 minute interval. The resultant mixture was agitated foran additional 105 minutes while the mixture was maintained below atemperature of 31 C. The pulp was then separated from the mixture andthoroughly washed. The pulp had a nitrogen content of 8.4 percent,equivalent to a D5. of 1.35. The pulp was slurried with water andrefined until all fiber clumps were broken up. The pulp was then furtherdiluted with water to a consistency of 0.05 percent and a uniform layerof pulp stock formed into a sheet on a conventional paper forming wire.The sheet was removed from the wire and a sample of the sheet tested ona Brecht Initial Wet Strength Tester. The sample as tested was a 42pound basis weight sheet (25 x 38500 basis) and had a moisture contentof 44 percent. The breaking length in meters was 4.2 meters. Sheets of100 percent cellulosic conventional papers having the same basis weightbut a moisture content of 80 percent were also tested and it was foundthat the breaking length in meters of a groundwood sheet was 2. meters,a bleached spruce kraft sheet 84 meters, and a bleached sulfite sheetwas 106 meters. Samples of the cyanoethylated cellulose web having amoisture content of 44 percent were then subjected to steaming for 5seconds and for 10 seconds. Samples of the resulting webs were againtested with the Brecht Initial Wet Strength Tester and it was found thatthe 5 second steaming operation had increased the breaking length of theweb to 49.6 meters. The moisture content of the steamed web was 36percent. The 10 second steaming operation had increased the breakinglength of the web to 71.2 meters and dried the web to a water content of32 percent. Samples of the steamed webs were completely dried and testedfor tensile strength in accordance with TAPPI Standard No. T-220. Bothwebs were found to have a tensile strength equivalent to 2.0 pounds permillimeter strip (42 pounds basis weight25 x 38500 sheet ream).

EXAMPLE 9 Cyanoethylated cellulose pulp having a nitrogen content ofabout 6.8 percent, equivalent to a D8. of 1.05

which had been prepared in accordance with the method described inExample 7 of the present specification was slurried with Water and allpulp knots broken up by a brief refining treatment. The pulp was dilutedto a consistency of 0.05 percent. A layer of pulp was formed on aconventional paper forming screen. This layer was then steamed for 10seconds. The resultant sheet of cyanoethylated paper wasself-sustaining. Tensile tests of the cyanoethylated paper in accordancewith TAPPI Standard No. T-220 showed that the sheet had a tensilestrength equivalent to 1.25 pounds per 15 millimeter strip (for a basisweight of 42 pounds25 x 38- 500). An unbeaten commercial bleached kraftpaper tested by the same method had a tensile strength of 1.65 poundsper 15 millimeter strip (for a basis weight of 42 pounds --25 x 38500).

A 57 pound basis weight sheet of cyanoethylated paper prepared as in thepresent example to be used as filter material was tested for porosity.It was found that a sheet which had a tensile strength of 2.2 pounds per15 millimeter test strip had a Frazier porosity of 173 cubic feet perminute per square foot under a standard pressure drop of /2 inch ofwater.

The cyanoethylated paper formed in this manner has the typicalcharacteristics of cyanoethylated cellulose in that it is very resistantto aging, and to attack by microorganisms. It is particularly useful asa filter material because webs of this type can be made withconsiderable porosity and with high thermal stability. It also may befurther treated as described in the present invention to form adielectric paper sheet of outstanding characteristics.

Paper because of its flexibility, low cost and outstanding dielectricproperties has found wide application as a dielectric material. It isvery commonly used as dielectric or insulating material in transformers,condensers, motors, cables, generators and related electricalcomponents. Paper has certain disadvantages when used as a dielectricmaterial, particularly its loss of mechanical strength and resiliencyupon aging. The exact nature of the aging of paper has not been fullydetermined but it is believed that it is probably a combination ofhydrolysis and oxidation of the cellulose molecule resulting in thesplitting of molecular chains and in the opening of glucose rings.

Paper has been made from slightly cyanoethylated cellulose fibers, i.e.having a nitrogen content of less than about 2.8 percent. It was foundthat this paper was a better dielectric than conventional paper, andthat the dielectric constants of the paper increased as the degree ofsubstitution by cyanoethyl groups increased. Cyanoethylated pulp havinga nitrogen content greater than about 2.8 percent however could not bebeaten and made into paper.

I have discovered that paper made from cyanoethylated cellulose fiberscan be processed to form sheet material having exceptional dielectricperformance and good resistance to aging. Broadly, the process of makingthe dielectric material comprises passing a moist cyanoethylated papersheet between a resilient non-metallic roll and a non-resilient metallicroll at a temperature of at least about 250 F., under a pressure of atleast about 10 pounds per linear inch and preferably above about 50pounds per linear inch. A single cyanoethylated paper sheet may beformed into a dielectric sheet in this manner or several sheets may bepassed through rolls simultaneously in a stack to form a laminateddielectric sheet.

The usual cellulosic substance employed in conventional papermaking maybe cyanoethylated to form the desired pulp. Although several methods maybe employed to cyanoethylate the raw cellulosic material, it isimportant that the carboxyl content of the resultant cyanoethylated pulpbe minimized and therefore pulp cyanoethylated in accordance with themanner previously described in the present specification is preferred.The

process may be applied to cyanoethylcellulose sheets of various degreesof substitution. It has been found that as the number of cyanoethylgroups substituted in pulp is increased, the dielectric constants ofpapers made from the resulting paper is increased. Dielectrics made frompaper having a D5. of 0.5-1.5 in accordance with the present process areparticularly desirable. The dielectric material may be made from websconsisting of cyanoethylated cellulose fibers or may be made from websof cyanoethylated cellulose fibers mixed with other materials such asnatural cellulose fibers, or synthetic materials such as nylon fibers,glass fibers, glass flakes and polyethylene terephthalate fibers(Dacron). The cyanoethylated pulp can be formed into a paper sheet orWeb as previously described.

The cyanoethylated paper sheet or stack of sheets entering the rollsshould have an initial moisture content of at least percent andpreferably about percent. Passage between the rolls of the moistenedsheet causes densification of the sheet and imparts a shiny surface tothe face of the sheet adjacent to the metallic roll. The sheet or stackis not ordinarily completely densified in a single pass through therolls but a series of passes through the rolls is made. Although thesheet or stack is moistened prior to the first pass of a series to theinitial moisture content of at least about 15 percent subsequent passesof the series are usually made Without remoistening until the desireddensification is attained, or substantially all Water is removed fromthe sheet or stack. If desired, the sheet may then be remoistened to a15 percent or more water content and a series of passes made through therolls with the face of the sheet reversed so that the previouslypolished side is now adjacent to the non-metallic roll. The sheet may,however, be passed through the rolls a number of times with one face ofthe sheet alternately adjacent to the resilient roll and then to thenon-resilient roll. When a blend of cyanoethylated cellulose andsynthetic fibers are used, fewer passes are normally required than whena pure cyanoethylated cellulose Web is used.

The metallic roll is normally maintained at a temperature of at leastabout 275 F. If the temperature is increased much beyond 450 F., thereis a tendency for the cyanoethylated cellulose to char. The temperaturemay however be limited to a lesser figure by the material of thenon-metallic roll. The metallic roll may be made of steel, bronze orother suitable metal. The resilient non-metallic roll is preferably of asynthetic material such as nylon but cotton filled rolls, paper rollsand other such resilient rolls may be used. Laminates may be prepared ofthe sheets by stacking a purality of the sheets together and passing thestack between the rolls. Laminates of considerable thickness can beformed in this manner.

Now that the process has been generally described it will be furtherillustrated by the following examples.

EXAMPLE 10 A cyanoethylated cellulose wood pulp was prepared asdescribed above. Two hundred and five parts of unbleached kraft pulp ofnorthern softwood, predominately spruce, was slurried with 1751 parts ofan 8.6 percent solution of sodium hydroxide for 10 minutes at roomtemperature. Five hundred and fifty parts of acrylonitrile was thenadded to the solution and mixing was continued for 90 minutes. Thetemperature of the reaction mixture was controlled so that the maximumtemperature was 49 C. The pulp was then separated from the mixture andthoroughly washed until all free sodium hydroxide and acrylonitrile wereremoved. The resultant cyanoethylated pulp had a nitrogen content of8.24 percent. The pulp was then slurried with water and given a 10minute refining treatment to break up fiber clumps. The pulp was thenslurried with water to a consistency of 0.05 percent to form a pulpstock. The stock was flowed onto a paper forming screen to form auniform layer of cyanoethylated cellulose fibers. This layer was drainedto a moisture content of 44 percent, and then steamed for 10 seconds.The steaming converted to stock layer into a self-sustaining sheet orWeb. Five of these webs were stacked together and this stack which had amoisture content of 32 percent was passed through the nip of a pair ofrolls. One of the rolls was a 9.5 inch diameter polished steel roll andthe other roll was an 8 inch diameter nylon roll. The surfacetemperature of the steel roll was maintained between 300 and 320 F.,during the rolling operation. The laminate was passed through the rolls30 times with the sheet being turned after each rolling operation sothat a face was alternately presented to the steel roll and then to thenylon roll. The pressure of the rolls on the laminate was approximately250 pounds per linear inch of the line of contact of the rolls. Theproduct obtained by this densifying treatment was a semi-transparentdense fibrous paperlike material having a basis weight of 225 pounds (25x 38500 sheets). The product had a flexibility which was equivalent tothat of an percent cotton content electrical grade paper laminate.

EXAMPLE II A cy-anoethylated cellulose pulp having a nitrogen content of6.37 percent was prepared from a bleached kraft pulp from northernsoftwoods, predominately spruce, in a manner similar to that describedabove. This cyanoethylated pulp was then converted into self-sustainingpaper sheets as described in Example 10. The sheets were dried. Four ofthe sheets 4 inches wide by 10 inches long were stacked and the stackWetted to a water content of 24 percent. The laminate was prepared bypassing the stack of 4 of these sheets through the nip of a steel-nylonpair of rolls. The surface temperature of the steel was maintained at280-290 F., and a pressure of pounds per linear inch was maintained onthe nip. The laminate was passed through the nip with the same side ofthe laminate presented to the steel roll on the first series of 6passes. The laminate was then remoistened and reversed for the secondseries of 5 passes. Several additional passes were then made.

The resistance to voltage breakdown of cyanoethylated fibrous paperlikematerials prepared in this manner is substantially greater than that ofconventional paper dielectrics. Furthermore, this high resistance tobreakdown is substantially retained in the dielectric material evenafter long periods of aging at high temperatures. In order to illustratethe improved dielectric qualities of the present product, 10 mil samplesof material produced in accordance with the method described in Example10 were compared with 16 mil samples of an 100 percent cotton contentelectrical grade paper as to mean value of voltage breakdown inkilovolts for both AC. and DC. potential. The second and third samplesof the cyanoethylcellulose (C.E.C.) laminate of the present inventionhad been aged at 300 F., for 48 hours and 216 hours respectively priorto the breakdown tests. Results of these measurements are as follows:

Table-Mean value of voltage breakdown in kilovolts This test illustratesthe resistance to aging even at relatively high temperatures of thematerial. Dielectric sheet material produced in accordance with theprocess of the present invention is relatively insoluble in organicsolvents. Thus the materials are particularly useful in motors which maybe exposed to commercial solvents such as the chloro-fiuoro-alkanes. Thedielectric materials are similar to paper in their mechanical propertiessuch as resistance to breakage during deformation. The rate of moisturedesorption from the material is exceptionally good and qualifies thismaterial for use as an insulating material in hermetically sealedmotors, where the insulation must be dehydrated before the motor issealed. A laminate 10 mils in thickness, made up of sheets of thecyanoethylated cellulose having a D5. of 1.37 was completely dehydratedin 30 minutes at 300 F.

The process of forming dielectric sheet material is usually applied toheat bonded cyanoethylated cellulose paper webs. It may however beapplied to a non-bonded layer of cyanoethylated cellulose fibers passedthrough at least the first set of rolls on a carrier;

What is claimed is:

1. The method of making a fibrous cyanoethylated cellulose sheetdielectric material which comprises treating cellulose fibers with anaqueous solution of a water soluble strongly basic hydroxide having a5-10 percent hydroxide concentration, reacting said cellulose fiberscontained in said solution with acrylonitn'le in an amount 85-300percent of the weight of the cellulose fibers at a temperature of lessthan about 50 C., to form cyanoethylated cellulose fibers having adegree of substitution of about 0.5-1.5 cyanoethyl groups per glucoseunit of the cellulose, separating the fibers from the solution, forminga layer consisting essentially of the cyanoethylated pulp having amoisture content of at least about percent, heating said layer whilesuppoited to convert said layer into a heat bonded self-sustaining papersheet, passing said sheet having an initial moisture content of at leastpercent between a resilient roll and a non-resilient roll at atemperature of from about 275 F. to 450 F. under a pressure of at leastabout 10 pounds per linear inch.

2. A process of forming a cyanoethylated cellulose material having adegree of substitution of about 0.5-1.5 cyanoethyl groups per glucoseunit of the cellulose, which comprises treating commercial wood pulpfibers in an aqueous solution of a water soluble strongly basichydroxide having a hydroxide concentration of about 5-10 percent, thenadding acrylonitrile to said solution in an amount of about 85-300percent of the cellulose present, the major proportion of said solutionremaining aqueous after the acrylonitrile addition, reacting theresultant mixture at a temperature less than about 50 C., to a degree ofsubstitution of about 0.5-1.5 cyanoethyl groups per glucose unit of thecellulose, and separating the cyanoethylated cellulose from the mixture.

3. The process of forming a cyanoethylated cellulose sheet whichcomprises forming a water moistened layer consisting essentially offibers of cyanoethylated cellulose having a degree of substitution of0.5-1.5 cyanoethyl groups per glucose unit of the cellulose, having amoisture content of at least about 10 percent, and heating the fibers,whereby the fibers are bonded into a paper sheet.

4. The method of forming. a cyanoethylated cellulose paper sheet fromcyanoethylated fibers having a degree of substitution of 0.5-1.5cyanoethyl groups per glucose unit of the cellulose, which comprisesforming a layer of the fibers on a paper forming wire from an aqueousslurry consisting essentially of said fibers, and steaming the layerwhile retaining a moisture content in said layer of at least 10% byweight, until said fibers are bonded into a sheet.

5. The method of forming a paper dielectric material which comprisespassing a fibrous sheet consisting essentially of cyanoethylatedcellulose fibers having a moisture content of at least 15 percent and adegree of substitution of 0.5-1.5 cyanoethyl groups per glucose unit ofthe cellulose between a resilient roll and a non-resilient roll at atemperature of between about 275 F. and 450 F., under a pressure of atleast about 10 pounds per linear inch.

6. The method of forming a cyanoethylated cellulose dielectric materialwhich comprises passing a layer of cyanoethylated cellulose fibershaving a degree of substitution of 0.5-1.5 cyanoethyl groups per glucoseunit of the cellulose and an initial moisture content of at least about15 percent between a resilient roll and a nonresilient roll under apressure of at least about 10 pounds per linear inch, and at atemperature of from about 275 F. to 450 F.

7. The method of forming a cyanoethylated cellulose paper dielectricmaterial which comprises assembling a stack of cyanoethylated cellulosepaper sheets, the cellulose fibers of said paper having a degree ofsubstitution of 0.5-1.5 cyanoethyl groups per glucose unit of thecellulose, the paper having an initial moisture content of at least 15percent and passing said stack between a resilient roll and anon-resilient roll at a temperature of from about 275 F. to 450 F., anda pressure of at least about 10' pounds per linear inch, to substantialdryness.

8. The process of forming a bonded fibrous sheet from a mixture ofnormally non-bonding fibers which comprises the steps of suspending inan aqueous slurry a mixture of fibers consisting essentially of at least5% cyanoethylated cellulose fibers having a degree of substitution of0.5-1.5 cyanoethyl groups per glucose unit of the cellulose andnon-hydrating fibers from the group consisting of glass,polyacrylonitrile, copolymers of polyvinylidene chloride and polyvinylchloride, polyethylene terephthalates, nylons, polyurethanes, and thenon-hydrating rayons, forming a layer of the fibrous mixture On aforaminous element, and heating the fibrous layer while in moistcondition until the fibers are bonded into a selfsustarining sheet.

References Cited in the file of this patent UNITED STATES PATENTS2,375,847 Houtz May 15, 1945 2,535,690 Miller et al. Dec 26, 19502,661,669 Friedrich Dec. 8, 1953 2,760,410 Gillis Aug. 28, 19562,793,930 Compton et al May 28, 1957 2,794,736 Cohen et al. June 4, 19572,930,106 Wrotnowski Mar. 29, 1.960

FOREIGN PATENTS 146,442 Australia May 12, 1952

1. THE METHOD OF MAKING A FIBROUS CYANOETHYLATED CELLULOSE SHEETDIELECTRIC MATERIAL WHICH COMPRISES TREATING CELLULOSE FIBERS WITH ANAQUEOUS SOLUTION OF A WATER SOLUBLE STRONGLY BASIC HYDROXIDE HAVING A5-10 PERCENT HYDROXIDE CONCENTRATION, REACTING SAID CELLULOSE FIBERSCONTAINED IN SAID SOLUTION WITH ACRYLONITRILE IN AN AMOUNT 85-300PERCENT OF THE WEIGHT OF THE CELLULOSE FIBERS AT A TEMPERATURE OF LESSTHAN ABOUT 50*C., TO FORM CYANOETHYLATED CELLULOSE FIBERS HAVING ADEGREE OF SUBSTITUTION OF ABOUT 0.5-1.5 CYANOETHYL GROUPS PER GLUCOSEUNIT OF THE CELLULOSE, SEPARATING THE FIBERS FROM THE SOLUTION, FORMINGA LAYER CONSISTING ESSENTIALLY OF THE CYANOETHYLATED PULP HAVING AMOISTURE CONTENT OF AT LEAST ABOUT 10 PERCENT, HEATING SAID LAYER WHILESUPPORTED TO CONVERT SAID LAYER INTO A HEAT BONDED SELF-SUSTAINING PAPERSHEET, PASSING SAID SHEET HAVING AN INITIAL MOISTURE CONTENT OF AT LEAST15 PERCENT BETWEEN A RESILIENT ROLL AND A NON-RESILIENT ROLL AT ATEMPERATURE OF FROM ABOUT 275*F. TO 450*F. UNDER A PRESSURE OF AT LEASTABOUT 10 POUNDS PER LINEAR INCH.